1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more particularly to an electrophotographic image forming apparatus using an electrophotographic photoreceptor formed of a charge transport layer overlying a charge generation layer and including at least a specific titanylphthalocyanine crystal, wherein a toner image is transferred with the application of not less than a specific current.
2. Discussion of the Background
Recently, information processing system apparatuses using an electrophotographic image forming method have been significantly progressed. Particularly, impressive improvements in print quality and reliability have been made in optical printers which optically record information by converting the information into a digital signal. This digital recording technology is applied not only to printers but also to conventional copiers, and so-called digital copiers have been developed. In addition, it has been predicted that the demand for copiers including digital recording technology in addition to conventional analog copying technology will increasingly grow because various information processing functions have been added thereto. Furthermore, because of the popularization of personal computers and the improvement of the performance thereof, digital color printers producing color images and documents have also rapidly progressed.
Recently, printers and copiers have been required to be smaller and to have high-speed printing capability. Accordingly, photoreceptors need to be smaller and rotate at a high speed. Therefore, an electrostatic latent image has to be developed to a toner image in a short time after the photoreceptor is irradiated, and thus deterioration of the electrical properties of the photoreceptor are accelerated because of repeated use due to an increase of the number of revolutions thereof.
To solve this problem, it is known that a highly sensitive titanylphthalocyanine crystal having at least a maximum diffraction peak at of a Bragg (2θ) angle of 27.2±0.2° when irradiated with a specific X-ray of CuKα having a wavelength 1.542 Å can be used as a charge generation material.
However, this crystal form is not stable as a crystal and easily changes due to mechanical stresses such as dispersion, and due to thermal stresses. The crystal form after the transition has substantially lower sensitivity compared to the original crystal form, and when a part of the crystal changes in form, sufficient photocarrier generation function is not realized. Furthermore, after repeated use of the photoreceptor, particularly after the application of a reverse charge by a transferer, the accelerated deterioration of the chargeability of the photoreceptor and abnormal images called background fouling tend to be produced.
In addition, because the frequency of image production significantly increases, it is essential that an apparatus produces high quality images. To achieve this, an electrostatic latent image having a high density has to be formed on the photoreceptor by a charger and an irradiator, subsequently the electrostatic latent image has to be faithfully developed by an image developer to form a toner image on the photoreceptor, and lastly the toner image on the photoreceptor has to be precisely transferred onto a transfer sheet. To achieve these, a method of forming an electrostatic latent image by high density writing with a small diameter beam as the irradiator, a method of forming a toner image which is faithful to the electrostatic latent image on the photoreceptor with a toner having a small particle diameter, and a method of faithfully transferring the toner image on the photoreceptor onto a transfer sheet by increasing the gap electric field strength to increase the transfer efficiency are available. Increasing the gap electric field strength particularly accelerates the deterioration of the electrical properties of a photoreceptor, causing abnormal images called background fouling, as mentioned above when the photoreceptor using the above-mentioned titanylphthalocyanine crystal having at least a maximum diffraction peak at of a Bragg (2θ) angle of 27.2±0.2° when irradiated with a specific X-ray of CuKα having a wavelength 1.542 Å, is repeatedly used.
On the other hand, the charge transport layer transporting a charge mainly includes a charge transport material and a binder resin, and is typically formed by coating a coating liquid in which these materials are dissolved or dispersed in a solvent. Specific examples of the solvent include halide solvents such as dichloromethane and chloroform having good solubility and applicability.
Lately, concern about environmental problems is growing and a photoreceptor using a non-halide solvent which does not seriously affect human bodies and the environment is desired. However, when a photoreceptor is formed using a charge transport layer coating liquid including this non-halide solvent, the optical attenuation properties of the photoreceptor deteriorate at a low electric field strength and the residual potential thereof increases. Particularly, this phenomenon noticeably occurs when a photoreceptor using the titanylphthalocyanine crystal showing uniquely a high sensitivity for a wavelength range of from 600 to 780 nm, stably emitted by the present LD and LED, and having at least a maximum diffraction peak at of a Bragg (2θ) angle of 27.2±0.2° when irradiated with a specific X-ray of CuKα having a wavelength 1.542 Å, is used without taking advantage of its primary properties as a charge generation material.
In addition, various methods of using non-halide solvents have been studied, and for example, Japanese Laid-Open Patent Publication No. 10-326023 discloses a method of using a dioxolane compound as an organic solvent excluding a halide. Furthermore, Japanese Laid-Open Patent Publications Nos. disclose methods of including a specific antioxidant or ultraviolet absorbent into a cyclic ether solvent such as tetrahydrofuran. However, even these methods do not have sufficient effect on the above-mentioned defects, or instead the additives deteriorate the sensitivity of the photoreceptor.
For these reasons, a need exists for an electrophotographic photoreceptor having a good optical attenuance, an electrophotographic image forming apparatus and a process cartridge for electrophotography using the electrophotographic photoreceptor, even when the titanylphthalocyanine having a specific high sensitivity is used as a charge generation material and a non-halide solvent is used for a charge transport layer coating liquid.